For the purposes of the present description, by electrical apparatus we mean, in general, any means or  electrical device generally being part of electrical installations in civil buildings and the like and usually intended to be mounted, for example built in, onto walls of such buildings.
This definition therefore includes, not limitingly, switches, electrical power sockets, sockets for data networks, TV sockets, telephone sockets, buttons, commutators, deviators, electrical adjustment devices in general, connectors, thermostats, timers, fuse holders, alarms/buzzers, emergency lights, for example removable, signalling lights, for example step markers, displays, for example LCD and the like.
As known, many of the aforementioned electrical apparatuses are usually installed on a wall using composite mounting structures, or groups of parts, generally including:                a box intended to be built into the wall;        an apparatus-carrying mounting frame (or support frame) fixable to the box and comprising a frame body developing about an opening defining a mounting seat suitable for receiving and holding one or more electrical apparatuses; and        a cover plate that can be removably fixed to the frame and provided with an opening for allowing a user to gain access, visually or manually, the electrical apparatuses installed on the support frame.        
In the prior art, several systems are known for removably fixing a cover plate to a support frame. For example, screw fixing systems are known, which despite being sturdy are almost outdated since they are unaesthetic.
Several snap-wise fixing systems between plate and frame also belong to the prior art. For example, it is known to provide a plurality of tongues on an edge portion of the plate intended for facing the support frame, adapted for engaging snap-wise with respective protruding edges provided on the external perimeter of the support frame.
The snap-wise fixing systems of the prior art, despite being highly used, in some cases exhibit a problem of fragility of the snap-wise fixing elements, in other cases are not such as to prevent an accidental detachment of the plate from the support frame.